European giants Bayern Munich and Barcelona have also been linked with a move for the 27-year-old.

After Dortmund’s 3-0 cup win over Hertha Berlin on Wednesday, Hummels told Germany’s Sky television: “It’s a very difficult decision, and when I finally make it people will understand how hard it is for me.

“It’s been taking me at least half an hour extra to fall asleep for several weeks now because this is such a big issue for me.”

He is not putting himself under any pressure to make up his mind, saying there is no deadline “because I fear that would not be easy to stick to”.

His father and agent, Hermann Hummels, says a return to Bayern Munich is not out of the question.

Hummels played for Bayern’s reserve team between 2005 and 2008 before moving to Dortmund, where his career has taken off.

“Mats is facing an enormously important decision in his career, in his life,” said Hermann Hummels.

“Dortmund are back among Europe’s best clubs and that is making the decision even harder for him.

“If he were to leave Dortmund, then he could only go to five or six top clubs, and of course Bayern are one of them.”